1. Technical Field
The present invention relates to a sintered valve guide that may be used in an internal combustion engine, and also relates to a production method for the sintered valve guide. Specifically, the present invention relates to a technique for further improving wear resistance of the sintered valve guide.
2. Background Art
A valve guide used in an internal combustion engine is a tubular component having an inner circumferential surface for guiding valve stems of an intake valve and an exhaust valve. The intake valve may be driven so as to take fuel gas into a combustion chamber of the internal combustion engine, and the exhaust valve may be driven so as to exhaust combustion gas from the combustion chamber. Therefore, the valve guide is required to have wear resistance and is also required to maintain smooth sliding conditions so as not to cause to wear the valve stems for long periods. Valve guides made of a cast iron are generally used, but valve guides made of a sintered alloy have recently come into wide use. This is because sintered alloys can have a specific metallic structure, which cannot be obtained from ingot materials, and therefore the sintered alloys can have wear resistance. Moreover, once a die assembly has been made, products having the same shape can be mass-produced, and therefore the sintered alloys are suitable for commercial production. Furthermore, a sintered alloy can be formed into a shape similar to that of a product, and thereby material yield can be high in machining. For example, a sintered alloy has a metallic structure in which an iron-phosphorus-carbon compound phase is precipitated and free graphite particles are dispersed in a pearlitic matrix. In this case, the pearlitic matrix is strengthened by adding copper and tin. Sintered valve guides made of this sintered alloy are disclosed in Japanese Examined Patent Publication No. 55-034858 and Japanese Patent Application of Laid-Open No. 4-157140. These sintered valve guides have been mounted in automobiles and have been commercially used as automobile valve guides by domestic and international automobile manufacturers.